1. Technical Field
The invention relates to an imaging lens system and an imaging apparatus, more particularly, to an imaging lens system suitable for use in a surveillance camera, a cell phone camera, an on-board camera, and the like employing an imaging device such as a CCD (Charge Coupled Device) and a CMOS (Complementary Metal Oxide Semiconductor), and an imaging apparatus having the imaging lens system.
2. Description of the Related Art
Recently, imaging devices such as CCD and CMOS have been downsized, and the number of pixels of the imaging device has been increased. In addition, an imaging apparatus main body having the imaging device has also been downsized, and thus it is also required to make an imaging lens system mounted therein be light and small.
Meanwhile, in an on-board camera, a surveillance camera, and the like, there is required a low-cost and high-performance lens that has high antiweatherability in the range of from outside air in a cold region to a vehicle compartment in summer of a tropical region and can be used in the wide temperature range.
JP Hei.11-271610 A describes a medium telephoto lens system that includes, in order from the object side, a positive first lens having a meniscus shape and having a convex surface directed toward the object side, a positive second lens, a negative third lens, an aperture diaphragm, and a cemented lens including a negative fourth lens and a positive fifth lens. Furthermore, JP Hei.5-224119 A describes a medium telephoto lens system that includes in order from the object side, a first lens group having three positive lenses having a meniscus shape and having a convex surface directed toward the object side, a second lens group having a negative lens formed in a meniscus shape, an aperture diaphragm, a third lens group having a cemented lens including a negative lens and a positive lens, and a fourth lens group having one or more positive lenses.
On-board cameras and surveillance cameras are used day and night, and particularly the on-board cameras take photographs using visible light in the daytime and take photographs using near-infrared light in the nighttime. Thus, these cameras need an optical system available in a wide wavelength range from the visible region to the near-infrared region. Furthermore, to photograph in low illumination intensity, these cameras also need a fast optical system having a large aperture ratio. In addition, considering that a cover glass, a filter, and the like are arranged between a lens system and an imaging device, an optical system having a long back focal length is preferred.
However, in the medium telephoto lens systems described in JP Hei.11-271610 A and JP Hei.5-224119 A, aberration correction in the visible region is considered, but performance of the lenses in the near-infrared region is not guaranteed. Furthermore, since the lens described in JP Hei.11-271610 A is formed as a slow optical system having an F number of about 4, the lens is inappropriate to use in the nighttime. Since the lens system described in JP Hei.5-224119 A has a large number of lenses, for example, seven or eight lenses, the lens system becomes a large optical system and becomes a high cost system. Moreover, since the lens system uses an aspheric surface lens, required precision at the time of cutting and assembly becomes higher, and thus the cost of the lens system becomes higher.